1. Field of the Invention
The present invention relates to a laser processing apparatus and a laser processing method, in which liquid is applied to a surface of a substrate such as a semiconductor wafer or a glass substrate for use in liquid crystal displays, the substrate covered with liquid is scanned with a laser beam, and the surface of the substrate is thereby processed.
2. Description of the Related Art
Methods of manufacturing semiconductor devices include steps of processing the surface of a substrate, by using a laser beam. Among these steps are: a step of processing a thin film such as an insulating film, a metal film or a resist film; a step of cutting dicing lines; a step of removing resist films from alignment marks before exposing the substrate to light (i.e., step of exposing the alignment marks). Jpn. Pat. Appln. KOKAI Publication No. 2002-224878, for example, describes a laser dicing process in which the surface of a substrate is scanned with a laser beam, thereby cutting dicing lines in the surface. Jpn. Pat. Appln. KOKAI Publication No. 2003-249427 discloses a process in which a laser beam is applied to a substrate before exposing the substrate to light, in order to remove a resist film covering alignment marks formed on the substrate, thereby to expose alignment marks. Laser beams are preferable for use in these processes, because they have a high energy density, making it possible to accomplish high-precision alignment.
Jpn. Pat. Appln. KOKAI Publication No. 2002-224878, mentioned above, discloses a laser processing apparatus. As FIG. 1 shows, the apparatus has a cup 11, a drive mechanism 12, and a chuck 13. The chuck 13 is provided in the cup 11. The drive mechanism 12 can rotates the chuck 13 and move the chuck 13 in the X- and Y-directions, positioning the chuck 13. The chuck 13 may hold a substrate 10, which has alignment marks and resist films covering the marks. A laser unit 14 applies a laser beam 18 to the resist films, removing the resist films from the substrate 10. A transparent plate 15 made of quartz glass is spaced a little from the substrate 10, facing the substrate 10. Pure water is kept flowing through gap between the substrate 10 and the transparent plate 15, supplied through an inlet port and drained from the gap trough an outlet port. This prevents dust formed in the laser dicing process from sticking to the surface of the substrate.
In most laser processes, a laser having a focal distance of about 50 μm is employed. If the work is placed at a distance shorter or longer than this focal distance, the laser dicing process cannot cut the resist films formed on the work. To cut dicing lines having a uniform depth, the laser needs to apply uniform energy to the work. Hence, the laser beam 18 should be focused at the surface of the substrate 10 in order to cut a dicing line in the surface of the substrate 10.
To focus the laser beam 18 at any point in the surface of the substrate 10, the substrate 10 must be located at a specific distance from the laser unit 14. To focus the laser beam 18 at any point in the surface of the substrate 10, and focus distances are uniformly adjusted to over the entire surface of the substrate, the surface flatness of the chuck 13 or that of the chuck-moving mechanism for moving the substrate 10 in the X- and Y-directions may be enhanced. If the chuck 13 and mechanism are machined at high precision to enhance their surface flatness, the manufacturing cost and the weight of the laser processing apparatus will inevitably increase.
Even if the chuck 13 and chuck-moving mechanism are improved in surface flatness, the apparatus is still disadvantageous because the chuck-moving mechanism has, for example, a ball screw that moves the chuck 13. When the ball screw is rotated, moving the chuck 13, its inclination angle varies, unavoidably inclining the surface of the chuck-moving mechanism. Further, even if the chuck 13 and chuck-moving mechanism are improved in surface flatness, the laser beam 18 cannot be focused at any point in the surface of the substrate 10, because the substrate 10 warps and has projections and depressions in its surface.